1. Field of the Invention
The present invention relates to a bucket elevator type continuous ship unloader.
2. Description of the Related Art
Cranes used to discharge bulk cargos such as coal, ore or grain from ships with a high degree of efficiency are generally called unloaders. Such unloaders are comprised of a glove bucket which grabs a cargo held in a hold and carries it out of the hold.
In recent years, however, bucket elevators have been replacing such glove buckets, offering improvements in the efficiency with which unloading can be undertaken.
These bucket elevator type continuous ship unloaders suffer from the problem that the scoop side of a bucket elevator strikes against the bottom of a ship that moves up and down during the unloading of a bulk cargo lying on the bottom, i.e., during a cleaning up operation, hence damaging the bottom of the ship and/or the buckets. Furthermore, the cargo remaining on the bottom must be collected during a cleaning up operation by means of, for example, a bulldozer that has to be carried into the hold, so that all the cargo can be carried out by the bucket elevator.
Under these circumstances, various proposals have been made with a view to meeting the increasing demand for an unloader that can obviate the above-described problems.
Japanese Utility Model Laid-Open No. 62-108339, for example, discloses a bucket elevator type continuous ship unloader which can be used for both a normal sidewise shovelling operation and a catenary cleaning up operation. In the former operation, the bulk cargo stored in a hold is shovelled by the bucket elevator which runs sidewise (in the direction perpendicular to the plane of FIG. 1, which illustrates a first embodiment of the present invention), with the bucket chain in shovel portion made taut in the horizontal direction. When the latter operation in which the bulk cargo remaining at the bottom is cleaned up is to be performed, the bucket chain is put in the form of a catenary (a loosened state) so as to enable any impact caused by the up-and-down movement of the ship bottom to be absorbed during the cleaning up of the cargo remaining at the bottom. In this type of continuous ship unloader, a front sprocket is mounted on the forward end of a pivot arm, and the bucket chain is made taut in the horizontal direction or given the form of a catenary by way of the pivot of the pivot arm. Further, a guide frame from under which the bucket chain can be inserted is provided, and in operation the bucket chain is inserted in the guide frame so that the force exerted on a bucket in a direction transverse to the direction of the bucket movement can be received by the guide frame.
However, in the above-described type of continuous ship unloader, since the guide frame receives only the force exerted on the buckets in a direction transverse to the direction of the bucket movement during the normal sidewise operation of shovelling, the bucket chain must be stretched tautly in order to make flat the surface of the cargo being shovelled by each bucket. This is because an uneven surface of the cargo would make its discharge unstable, particularly in the case of a large unloader in which a cargo is shovelled over a long distance. This requires a bucket chain which is not only rigid enough to ensure the tension required to pull a large number of buckets full of cargo, but is also able to stretch the shovel portion tautly. This means that the bucket chain is heavy, which is undesirable for a ship unloader with a cantilever boom. In addition to this, when the cargo remaining at the bottom has to be cleaned up, shovelling starts with a bucket located in the vicinity of the front sprocket mounted on the forward end of the pivot arm. This does not allow for a sufficient sag (downward bending of a bucket chain under its own weight), and therefore cannot eliminate the possibility of impact being caused by the up-and-down movement of a ship's bottom.